Tight tolerance zero crossing detector circuit

Abstract

A zero crossing detector provides an optically coupled output signal with a leading edge which occurs in the vicinity of the zero crossing point of an AC voltage waveform but no later than the zero crossing time. The circuit input includes a filter and rectifier. The filtered and rectified voltage is then applied to a complex impedance circuit with an input and an output, which provides an output lagging its input. A switching device, which may be a light emitting diode, is coupled between the output of the impedance circuit and the rectifier. Thus, when the lagging output voltage of the impedance circuit exceeds the output of the rectifier, the switching device conducts, producing the output signal.

What is claimed is: 
     
       1. A zero crossing circuit for producing a signal with a transition in the vicinity of, but no later than, the zero crossing point of an AC voltage comprising: rectifying means with an input and an output coupled to said AC voltage providing at said output a rectified replica of a voltage applied at said input,   impedance means with an input coupled to the output of said rectifying means, and an output, said impedance means providing at said output a voltage lagging a voltage applied to said impedance means input, and   switching means comprising a diode coupled across said output and input of said impedance means for producing said signal as said diode is biased into conduction prior to a minimum in the output of said rectifying means by a difference in potential between said impedance means output and input said switching means producing a signal when and so long as said impedance means output exceeds said impedance means input.   
     
     
       2. The apparatus of claim 1 in which said signal is photo coupled and in which said diode in said switching means comprises a radiation emitting solid state device responsive to a difference in said impedance means output and input; a radiation sensitive solid state device responsive to said radiation emitting solid state device, coupled across a power supply and switched from a non-conducting to a conducting state when said radiation emitting solid state device emits radiation. 
     
     
       3. The apparatus of claim 1 in which said impedance means comprises in series a first resistor and a capacitor coupled across said rectifying means output and a discharge circuit for said capacitor also coupled across said rectifying means output. 
     
     
       4. The apparatus of claim 3 in which said switching means comprises a radiation emitting solid state device. 
     
     
       5. The apparatus of claim 4 wherein said switching means further includes a rectifier coupled in series with said radiation emitting device and a leakage current resistor coupled across said radiation emitting device. 
     
     
       6. The apparatus of claim 4 in which said radiation emitting device is an LED. 
     
     
       7. The apparatus of claim 1 which further includes line voltage filter means coupled between said rectifying means input and a source of said AC voltage. 
     
     
       8. The apparatus of claim 3 in which said first resistor has greater resistance than said discharge circuit. 
     
     
       9. A zero crossing detector to provide a signal in the vicinity of, but no later than, the zero crossing point of an AC voltage waveform comprising: a full wave rectifier with an input and output,   a series RC circuit coupled to the output of said rectifier with a resistance R 4  and a capacitor with capacitance C,   a partial discharge circuit for said capacitor C with resistance R 3  also connected to the output of said rectifier and,   switching means for producing said signal coupled between said capacitor and said partial discharge circuit, said switching means producing a detectable output when a voltage on said capacitor exceeds an output voltage from said rectifier, which further includes:   an optical coupling for said signal and wherein said switching means comprises a radiation emitting diode and a radiation sensitive active device for producing a voltage excursion on the sensing of radiation emitted by said radiation emitting diode.   
     
     
       10. The apparatus of claim 9 which further includes a line voltage filter coupled between a source of said AC voltage and said full wave rectifier. 
     
     
       11. The apparatus of claim 10 in which a product of said resistance R 4  and capacitance C is greater than the period of said AC voltage waveform. 
     
     
       12. The apparatus of claim 10 in which a product of said resistance R 3  and said capacitance C is less than the period of said AC voltage waveform. 
     
     
       13. A zero crossing detector for producing a signal timed in relation to a zero crossing of an AC voltage comprising: a full wave rectifier with input and output,   a line filter coupled between said AC voltage and said full wave rectifier input,   complex impedance means, with an input coupled to said full wave rectifier output and an output for producing an output voltage lagging a voltage coupled at said input of said complex impedance means, said complex impedance means including a capacitor across which said lagging voltage is produced and a charging circuit coupled thereto for charging said capacitor, and   switching means comprising a diode coupled across said output and input of said complex impedance means for producing said signal as said diode is biased into conduction prior to a minimum in the output of said full wave rectifier by a difference of potential between output and input of said complex impedance means,   whereby time delays occasioned by said line filter are prevented from influencing timing of said signal by selection of parameters of said charging circuit.   
     
     
       14. The apparatus of claim 13 wherein said diode comprises a radiation emitting solid state device, and wherein said switching means further includes a radiation sensitive solid state device responsive to radiation emitted by said radiation emitting solid state device and coupled across a power supply and switched from a non-conducting to a conducting state when said radiation emitting solid state device emits radiation. 
     
     
       15. The apparatus of claim 13 in which said complex impedance means comprises, in series, a first resistor and said capacitor coupled across said output of said rectifying means, and a discharge circuit for said capacitor including a second resistor also coupled across said output of said rectifying means.